The present invention relates to methods and apparatus for determining a position of an object in relation to incident radiation. In particular, although not exclusively, some embodiments of the invention may be used to improve a quality of data relating to the object or the radiation.
WO 2005/106531, which is incorporated herein by reference for all purposes, discloses a method and apparatus of providing image data for constructing an image of a region of a target object. Incident radiation is provided from a radiation source at the target object. An intensity of radiation scattered by the target object is detected using at least one detector. The image data is provided responsive to the detected radiation. A method for providing such image data via an iterative process using a moveable probe function is disclosed. The methods and techniques disclosed in WO 2005/106531 are referred to as a ptychographical iterative engine (PIE).
PIE provides for the recovery of image data relating to at least an area of a target object from a set of diffraction pattern measurements or to the determination of information associated with radiation illuminating the target object. Several diffraction patterns are recorded at a measurement plane using one or more detectors, such as a CCD or the like.
WO 2010/064051, which is incorporated herein by reference for all purposes, discloses an enhanced PIE (ePIE) method wherein it is not necessary to know or estimate a probe function. Instead a process is disclosed in which the probe function is iteratively calculated step by step with a running estimate of the probe function being utilised to determine running estimates of an object function associated with a target object.
Other methods are known which are referred to a coherent diffraction imaging (CDI) which are based on the measurement of scattered radiation, such as that by P Thibault, Dierolf, et al entitled “Probe Retrieval in Ptychographic Coherent Diffractive Imaging” disclosed in Ultramicroscopy, 109, 1256-1262 (2009) and WO2011033287 entitled “Method And Apparatus For Retrieving A Phase Of A Wavefield”, which is herein incorporated by reference, by the present inventor.
In some of the above methods, particularly the ptychography methods, movement between a plurality positions is often required i.e. to record a plurality of diffraction patterns with the illumination located differently with respect to the object. In some methods, the movement is relative movement between the object and the radiation or probe. To achieve such movement, the object, probe or both may be moved or translated amongst a plurality of positions. The probe may be moved by altering a position of the radiation source, focussing optics or an aperture. The object may be moved upon, for example a translation stage. The accuracy of such movements presents a limitation on the accuracy of the resultant image data.
In other methods, particularly the coherent diffraction imaging methods, only a single diffraction pattern is required to be recorded. However, it is difficult to eliminate drift of the illuminating radiation and/or the object to within the accuracy required by such methods.
Embodiments of the invention aim to improve a determination of a position based on recorded intensity measurements.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.